Cosmic high-energy gamma rays are produced by a diverse array of sources in the universe that signal the presence of natural particle accelerators. Among these are distant cores of active galaxies and the mysterious gamma-ray bursts, pulsars and cosmic-ray interactions in our Galaxy, and the Sun and Moon in our solar system. The recently launched Fermi Gamma-ray Space Telescope, with unprecedented sensitivity and angular resolution, is discovering hundreds of new sources of cosmic gamma rays with the hope of probing the inner workings of the high-energy accelerators and possibly uncovering the nature of dark matter.